Dual search mode type tuning system

ABSTRACT

A signal seeking tuning control system of a television receiver is first caused to operate in a first mode to search a frequency range surrounding a selected channel in accordance with predetermined conditions of a picture synchronization signal, and thereafter caused to operate in a second mode to search essentially the same frequency range but this time in response to predetermined conditions of an automatic fine tuning signal.

The present invention pertains to the field of signal seeking tuningsystems for television receiver in which the tuning control is changedduring a search mode of operation until a proper carrier is located.

There are a vast variety of such signal seeking tuning systems. Mostsignal seeking tuning systems employ a detector which examines thecondition of an automatic fine tuning (AFT) signal which isconventionally produced in a television receiver to indicate the degreeof mistuning of the picture carrier of the IF signal. However, the AFTsignal may respond to undesired signals in the same manner that itresponds to a picture carrier. Accordingly, the AFT signal is not alwaysa reliable indication of when a channel is properly tuned.

Accordingly, many signal seeking tuning systems employ detectors whichdetermine that both an automatic fine tuning (AFT) signal and a picturesynchronization component have predetermined conditions in order todetermine when a picture carrier, rather than some undesired signal, hasbeen tuned. In order for the search to be terminated, both the AFTsignal and the picture component must have predetermined conditionsassociated with the proper tuning of the picture carrier. With theincreased use of cable television master antenna installations, signalseeking tuning system have become of more interest to manufacturers oftelevision receivers because the RF carrier produced by suchinstallations may be considerably translated in frequency in comparisonto respective broadcast carriers which have standard frequencies. Insome cable television and master antenna installations, so-called musicchannels may be provided in which the sound carrier is modulated but thepicture carrier is not. This results in the absence of picturesynchronization components. Accordingly, such music channels may not betuned in a signal seeking tuning system which requires the AFT andpicture synchronization signals to simultaneously have predeterminedconditions indicative of a properly tuned channel.

In accordance with one aspect of the present invention, a signal seekingtuning system has mode control apparatus which causes a range offrequencies to be searched twice in succession, first in response to apicture synchronization component and second in response to an AFTsignal.

The present invention will be described with reference to theaccompanying drawing in which:

FIG. 1 shows partially in block diagram form and partially in logicdiagram form an embodiment of the present invention as it may beemployed in a phase locked loop (PLL) tuning system of a televisionreceiver;

FIG. 2 shows a graph which illustrates the operation of the tuningsystem shown in FIG. 1;

FIG. 3 shows a waveform useful in understanding the operation of thetuning system of FIG. 1; and

FIGS. 4, 5a, 5b and 6 show logic implementations of portions of thetuning system shown in FIG. 1; and

FIGS. 4a and 5c show waveforms useful in understanding theimplementations of FIGS. 4, 5a and 5b, respectively.

In the television receiver shown in FIG. 1, a voltage controlled RFsection 1, voltage controlled local oscillator (LO) 3, mixer 5, IFsection 7, picture processing unit 9, picture synchronization unit 11,deflection unit 13, picture tube 15, sound processing unit 17 andspeaker 19 are arranged in conventional manner. Synchronization unit 11derives a so-called composite synchronization signal including bothhorizontal and vertical synchronization pulses from the IF signal. Inresponse to these pulses, deflection unit 13 generates horizontal andvertical electron beam deflection signals for picture tube 15. Afrequency discriminator 21 generates an AFT signal representing thedegree and sense of deviation between the frequency of the picturecarrier of the IF signal and its nominal value, e.g., in the UnitedStates, 45.75 MHz.

RF carriers are provided by an RF source 23, which may be an antenna,cable or master antenna installation, or video accessory like a videocamera, video cassette player or video disc player.

The remaining portion of the receiver shown in FIG. 1 is a tuningcontrol system for generating the tuning voltage for RF section 1 and LO3. It includes a fixed divider or prescaler (÷K) 25, a programmabledivider (÷N) 27, a crystal reference oscillator 29, a fixed divider (÷R)31, a phase comparator 33 and a low pass filter (LPF) 35 coupledtogether with LO 3 in a phase locked loop configuration. The factor N bywhich programmable divider 27 divides is set in accordance with adigital word representing the channel number of the channel selected bymeans of channel selector 37.

When a switch 39, hereinafter referred to as the PLL/AFT mode switch forthe reasons discussed below, is in a PLL state, the output of phasecomparator 33 is coupled to the input of LPF. In this configuration, theLO frequency is set by phase locked loop operation in accordance withthe value of N and the number of pulses removed from the input ofprogrammable divider 27 by a pulse-swallower 41, the function of whichwill be described below. When PLL/AFT mode switch 39 is in an AFT state,the AFT signal generated by AFT discriminator 21 is coupled to the inputof LPF 35. In this condition, the LO frequency is controlled in responseto the AFT signal.

After a new channel is selected, channel selector 37 generates a"change" pulse to which a PLL/AFT mode control unit 43 responds to causeswitch 39 to couple the output of phase comparator 33 to LPF 35 andthereby initiate PLL operation. As will be explained below, pulseswallower 41 is controlled to change the LO frequency in step-wisefashion. During each frequency step, a lock detector 45 coupled to phasecomparator 33 generates a lock pulse when the phase deviation betweenthe two input signals of phase comparator 33 has a predetermined lowvalue. This signifies that the PLL operation for that step has beencompleted.

Under certain conditions to be detailed below, PLL/AFT mode control unit43 causes switch 39 to couple the AFT signal to LPF 35. When LO 3 isunder AFT control, an offset detector 47 counts the number of periods ofthe ÷N output signal during a reference interval to compare thefrequency of the LO signal to the value established during the precedingPLL operation. If the LO frequency is offset, e.g., by 1.25 MHz, fromthe value established during the preceding PLL operation, PLL operationis again initiated and the LO frequency is stepped by controlling pulseswallower 41.

A phase locked loop tuning system of this general type is described inU.S. Pat. No. 4,031,549, entitled "Television Tuning System withProvisions for Receiving RF Carriers at Non-Standard Frequencies",issued in the names of Rast et al. on June 21, 1977. A tuning system ofthe type described in this U.S. patent is embodied in CTC-108 typetelevision chassis manufactured by RCA Corporation of Indianapolis,Indiana and documented in "RCA Television Service Data--CTC 108", File1980 C-5. That tuning system contains portions closely corresponding torespective portions of the present tuning system (with the exception ofpulse swallower 41) so far described. Accordingly, the above-identifiedpatent and RCA publication are incorporated by reference.

As earlier indicated, pulse swallower 41 selectively removes pulses fromthe input signal of programmable divider 27 and thereby step-wisechanges the LO frequency. Specifically, in response to the digital wordrepresenting the channel number generated by channel selector 37, N isset to a value 3 less than the value of the nominal local oscillatorfrequency in MHz. Thus, for example, if channel 2 is selected, for whichthe frequency in the United States is 101 MHz, N is set to 98. Sinceprogrammable divider (÷N) 27 generates 1 pulse at its output for every Npulses at its input, by removing (i.e., "swallowing") input pulses toprogrammable divider 27, the LO frequency is increased. The number ofpulses swallowed and the rate at which the pulses are swalloweddetermine the size of the step.

A binary rate multiplier (BRM) 49 controls the number of pulsesswallowed and the rate at which they are swallowed. BRM 49 is a counterwhich generates a number of output pulses during the period of a clocksignal applied to its clock (C) input which is directly proportional tothe number represented by the digital word coupled to its control inputsfrom a step control counter 51. The frequency of the clock signal (1/2R)of BRM 49 is selected to be 1/2 of the frequency of the referencefrequency signal (R) coupled to phase comparator 33 from fixed divider31 (÷R). Since the operation of the PLL causes the frequencies of theoutput signals of programmable divider 27 and fixed divider 31 to beequal, for each pulse swallowed by pulse swallower 41, the LO frequencyis increased by 1/2 MHz.

Step control counter 51 is a down counter, the contents of which areentered from a "count=9 ROM" 53, a "count=14 ROM" 55, or a "1 MHz stepcontrol counter" 57, and which are decreased in response to theapplication of a clock signal to its clock (C) input. Step controlcounter 51 is a 4-bit counter. Accordingly, the digital word generatedat its output represents a number between 0 and 15. Since for eachchannel selected, N is set 3 lower than the respective nominal value forthe selected channel, each of the numbers corresponds to a respective LOfrequency step with respect to the nominal LO frequency for the selectedchannel as follows.

    ______________________________________                                        CONTENTS         LO FREQUENCY STEP                                            O COUNTER 51     (IN MHZ)                                                     ______________________________________                                        0                -3                                                           1                -2.5                                                         2                -2                                                           3                -1.5                                                         4                -1.0                                                         5                -0.5                                                         6                0 (nominal)                                                  7                +0.5                                                         8                +1.0                                                         9                +1.5                                                         10               +2.0                                                         11               +2.5                                                         12               +3.0                                                         13               +3.5                                                         14               +4.0                                                         15               +4.5 (not used)                                              ______________________________________                                    

As indicated, a count of 6 corresponds to the nominal LO frequency. Acount of 15 corresponds to a step of +4.5 which, as will be explainedbelow, is not used.

Desirably, each pulse generated by BRM 49 should remove only one pulsegenerated by prescaler (÷K) 25. The logic arrangement for this purposeand for swallowing or removing pulses generated by prescaler 25 from theinput of programmable counter (÷N) 27 is shown in FIG. 4.

The CD4089 manufactured by RCA Corporation, Somerville, N.J., is anintergrated circuit including a binary rate multiplier of the typesuitable for use as BRM 49. In that integrated circuit, the countercorresponding to counter 51 for controlling the number of output pulsesthe binary rate multiplier produces is included with the binary ratemultiplier structure itself. Accordingly, while counter 51 is shownseparately for purposes of explanation, it will be appreciated that itmay be part of BRM 49. In addition, the use of a pulse swallower forcontrolling the frequency of a local oscillator in a PLL is described ina magazine article entitled "Introduction to Microcomputer ControlledRadio Tuning System", by Schillhof, appearing in "Electronic Componentsand Applications", Vol. 1, No. 4, August 1979.

When a new channel is selected to properly locate and tune the picturecarrier, the present tuning system first searches a first range offrequencies surrounding the nominal frequency for the selected channelby, in step-wise manner, using the composite synchronization signal andthereafter searches a second range of frequencies surrounding thenominal frequency by using the AFT signal. This is indicated in thegraph shown in FIG. 2. As shown, the first and second ranges have asubstantial overlapping or common portion, the first range extendingfrom +4 MHz to -3 MHz of the nominal frequency and the second rangeextending from approximately +3.25 MHz to -4.25 MHz of the nominalfrequency. In each search operation, the contents of step controlcounter 51 are changed in steps to effect corresponding changes of theLO frequency. Since, as will be explained below with reference to FIG.3, the AFT has been found to provide an unreliably accurate indicationof the presence and location of a picture carrier for a selectedchannel, a synchronization (hereinafter "sync") presence detector 61examines several parameters of the composite sync signal. If all theparameters are within predetermined tolerance ranges, sync presencedetector 51 generates a high logic level ("1") and otherwise generates alow logic level ("0"). An arrangement for this purpose is shown in FIGS.5a and 5b.

However, the generation of a "1" by sync presence detector 61 alone doesnot reliably indicate the presence and location of the picture carrierfor the selected channel. The reason for this, as well as for theunreliability of the AFT signal for the latter purpose, may beunderstood with reference to FIG. 3.

FIG. 3 is a graphical representation of both the AFT signal and theoutput signal of sync presence detector 61 (hereinafter "sync presence"signal) as the LO frequency is increased, thereby causing the frequencyof the IF carriers to correspondingly increase. In FIG. 3: "X"corresponds to the selected channel; "X-1" corresponds to the loweradjacent channel in the RF range (noting that f_(IF) =f_(LO) -f_(RF));and "X+1" corresponds to the higher adjacent channel in the RF range.The term "PIX" corresponds to the picture carrier. The term "CHROMA"corresponds to the color carrier. The term "SND" correspond to the soundcarrier. It will be noted that spurious signals (due to undesired crossmodulation products produced in mixer 5) and sound carriers produce asimilar AFT response to that of the desired picture carrier of theselected channel. Accordingly, the AFT signal is an unreliableindication of the presence and location of the picture carrier. Withreference to solid line waveform A, which represents the sync presencesignal when IF section 7 employs a picture synchronous demodulator, itis noted that the level of the sync presence is a "1" for sound carriersas well as for the picture carrier. With reference to broken linewaveform B, which represents the sync presence signal when IF section 7employs an envelope detector, it is noted that the level of the syncpresence signal is a "1" for spurious and sound carriers as well as forthe picture carrier. Accordingly, the "1" level of the sync presencesignal reliably indicates the presence but not the accurate location ofthe picture carrier.

To accurately determine the location of the picture carrier, in thepresent tuning system, a search is conducted by decreasing the frequencyof the IF signal by decreasing the frequency of the LO signal and usinga sync presence transition detector 63 to determine when the syncpresence signal changes from a "0" to a "1". As is indicated in FIG. 3,when the searching occurs in the decreasing frequency direction, the"01" transition (hereinafter "01X") accurately determines when thepicture carrier of the selected channel is in the AFT control range. Ithas been found that the "01X" occurs slightly higher than the nominalpicture carrier frequency, which occurs at the center level of the AFTsignal, by approximately 0.7 MHz. Accordingly, if a "01X" is detectedduring the sync transition search operation, the LO frequency isdecreased by a 0.5 MHz step as will be explained below. (If the searchwere conducted in the increasing frequency direction, a "10X" would besought. However, after the latter was found, the LO frequency wouldstill decreased by 0.5 MHz.)

"If a "01X" is obtained during a given LO frequency step, a "stepfreeze" signal is generated. In response, no further stepping occurs andPLL/AFT mode control unit 43 causes switch 39 to apply the AFT signal toLPF 35 so as to fine tune the LO frequency. If a "01X" is not obtained,the logical complement of the "step freeze" signal, i.e., "step freeze",is generated and applied to the clock (C) input of step control counter51 through an "and" gate 65, enabled by the "lock" pulse. In response,the LO frequency is decreased by 0.5 MHz to the next step.

Sync presence detector 61 is reset in response to the application of the"change" pulse to its reset (R) input. Transition detector 63 is set toa predetermined initial state in response to the application of the"change" pulse or a signal generated by "count=15" detector 69, thefunction of which will be described below, to the reset (R) inputthrough an "or" gate 67. As will be explained below, transition detector63 is set and reset under certain conditions in response to theapplication of signals to its set (S) and reset (R) inputs,respectively, and utilizes the "lock" pulse, applied to its clock (C)input, as a clocking signal. A logic arrangement for sync presencetransition detector 63 is shown in FIG. 6.

The remaining portion of the tuning system shown in the lower right-handportion of FIG. 1 is the logic for selectively enabling the two searchoperations and for controlling the step sequence in the sync presencetransition search operation. Those operations can best be understood byconcurrent reference to FIGS. 1 and 2.

A "Sync/AFT search mode control unit" 71, simply comprising a set-resetflip-flop (SR FF), is responsive to the "change" signal coupled to itsset (S) input to generate a "1" at its Q output, hereinafter referred toas the "sync search enable" signal. That signal is coupled to enabling(E) inputs of ROMs 53 and 55. When enabled, upon application ofrespective read signals to respective read (RE) inputs of the ROMs, theyapply a respective 4-bit digit representing a count to step controlcounter 51. This in turn causes the LO frequency to assume acorresponding value as specified in the above table. For this purpose,ROMs 53 and 55 each may simply comprise four transmission gates (i.e.,semiconductor switches) for selectively coupling respective logiclevels, which in binary coded format represent the associated count, tostep control counter 51.

Specifically, in response to the "change" signal (which is a pulse), ROM53 binary signals representing a count of 9 are entered into stepcontrol counter 51. This causes the LO frequency to be set to a value+1.5 MHz above its nominal value. This initial step is selected becauseit is assumed that the RF carriers for many channels will be at or verynear their standard values. In this case, with a +1.5 MHz step, thesynchronization components of the IF signal will be removed by theconventional lower adjacent channel sound trap, e.g., in the UnitedStates at 47.25 MHz, thereby ensuring that the sync presence signal willbe a "0". Assuming the RF carrier is at or very near its standardfrequency and the "01X" occurs at about 0.7 MHz above the picturecarrier, when the LO frequency is decreased by 0.5 MHz, as it will besince no "step freeze" would have been generated at the +1.5 MHz step,the sync presence signal remains a "0". However, on the next step, thesync presence signal becomes "1". In response to the resulting "01X", a"step freeze" signal is generated, preventing further steps and ensuringthe AFT signal to be applied to LPF 35.

If the frequency of the RF carrier is translated from its standard valueby a large positive offset (which corresponds to a large negative offsetof the IF signal), e.g., 2 MHz, at the initial +1.5 MHz step, it islikely that the sync presence signal will be at its "1" level.Therefore, it would be logical to start the search at a large positiveoffset of the LO signal, e.g., at a +4 MHz step as indicated in thedotted line portion of FIG. 2. To accomplish this, the Q output of a SRFF 73 is set to a "1" (identified as the "+1.5" signal) in response tothe "change" signal to indicate that the LO frequency is at the +1.5step. At this point, if the sync presence signal is at its "1" level,when the "lock" signal (a positive-going pulse) is generated, an "and"gate 75 generates a "1" (identified as the "+1.5, sync `1`" signal) toindicate that at the +1.5 step the sync presence signal was at its "1"level. That "1" is coupled through an "or" gate 77 to the read enable(RE) input of ROM 55 and also to the set (S) input of an SR FF 79. Inresponse, ROM 55 enters a count of 14 into step control counter 51 whichcauses the LO frequency step to be +4.0 MHz. In addition, the Q outputof FF 79 is set to a "1" (identified as the "+4.0" signal) to indicatethe step sequence starting from +4.0 MHz rather than from +1.5 MHz. Thisinformation is used later, as will as be explained below.

Whether the sync transition search starts at the +1.5 or +4.0 step, whenthe "lock" signal is generated at the completion of each step, if the"01X" has not been generated, in response to the "step freeze" signalthe contents of step control counter 51 are decreased by a count of 1.Accordingly, the LO frequency is decreased by 0.5 MHz. This processcontinues until the "01X" is detected and the "step freeze" signal isgenerated, in which case the search is stopped and the AFT signal isapplied to LPF 35, or the end of the search range.

To detect the latter occurrence, a "count=15" detector 69 is coupled tothe outputs of step control counter 51. The -3 MHz step corresponds to acount of 0. However, since if the "01X" is not detected during the -3MHz step, which corresponds to a count of 0, the count is againdecreased in response to the "step freeze" signal, the next count, whichis 15 (i.e., the 4-bit digital word changes from 0000 to 1111), isdetected rather than the 0 count. Detector 69 generates a "1" when acount of 15 is detected. For this purpose, detector 69 may simplycomprise a four input "and" gate.

If the Q output signal of FF 79 is a "1", indicating that the searchstarted at +4.0, an "and" gate 81 generates a "1" in response to the "1"generated by detector 69. This "1" signifies that the range for the syncpresence transition search has been completely searched and is coupledthrough an "or" gate 83 to the reset (R) input of sync/AFT search modecontrol unit 71. This causes the Q of search mode control unit 71(identified as the "AFT search enable" signal) to become a "1". As aresult, the sync presence transition search mode is terminated and theAFT search mode, to be described below, is initiated.

The "1" generated by detector 69 is coupled to the S input of a SR FF 85in response to which its Q output signal is set to a "1" (identified asthe "-3.0" signal) to indicate that the end of the sync presencetransition search range has been reached for future use as will bedescribed below.

The "1" generated by detector 69 is also coupled through "or" gate 77 tothe read (RE) input of "count=14" ROM 55. This sets the contents of stepcontrol counter 51 to 14. The purpose of this is to establish restart ofthe sync presence transition search mode at the +4.0 MHz step if itpreviously started only at the +1.5 MHz step as is indicated by a "0" atthe Q output of FF 79. If the latter is true, "and" gate 81 would nothave been enabled to generate a "1" in response to the "1" generated by"count=15" detector 69 as described above. Accordingly, if the Q outputof FF 79 is a "0", the sync presence transition search will be enabledto restart the search at the +4.0 step.

The purpose of restarting the search at the +4.0 step is to cover theportion of the range between the +4.0 and +1.5 step which will not havebeen covered if the sync presence transition search initially started atthe +1.5 step. However, since the search starting at the +1.5 stepcovered the portion of the range between the +1.5 and -3 steps, thesearch restarted at the +4.0 step need only go to the +1.5 step. To thisend, a "count=8" detector 87 is employed to determine when the binarysignals generated by step control counter 51 represent a count of 8.Although a count of 8 corresponds to a step of +1.0 rather than +1.5, itwill be recalled that if the "step freeze" signal is not generatedduring the +1.5 step which corresponds to a count of 9, the count willbe automatically decreased to a count of 8. Accordingly, a count of 8properly indicates that the poriton of the range between +4.0 and +1.5has been searched.

When a count of 8 is detected, detector 81 generates a "1". Since theportion of the range between the +1.5 and -3.0 steps was previouslysearched, the Q output of FF 85 will be set at a "1". Accordingly, an"and" gate 89 is enabled to generate a "1" in response to the "1"generated by detector 87. The latter "1" is coupled through "or" gate 83to the reset (R) input of search mode control 71 to end the syncpresence transition search operation and initiate the AFT searchoperation by generating the "AFT search enable" signal.

During the sync presence transition mode, FF 73 is reset in response tothe "lock" pulse. FFs 79 and 85 are reset in response to the "change"pulse.

By the end of the sync presence transition search mode, channels havingpicture carriers providing a correct composite sync signal will havebeen located and tuned. However, as earlier noted, some channels, suchas music channels, do not have modulated picture carriers and thereforewill not have a correct composite sync signal. Such channels are locatedand tuned during the AFT search mode as follows.

The "AFT search enable" signal is coupled to the set (S) input of syncpresence transition detector 63 which causes the "step freeze" signal tobe generated. The "AFT search enable" signal is also coupled to the read(RE) input of 1 MHz step control counter 57 which causes its contents tobe entered into step control counter 51. Counter 51 is reset to apredetermined state corresponding to a count of 6 in response to the"change" pulse applied to its reset (R) input. Accordingly, in responseto the "AFT search enable" signal, a count of 6 is entered into stepcontrol counter 51 which causes the nominal LO frequency to beestablished by PLL operation.

Thereafter, when the "lock" pulse is generated, PLL/AFT mode controlunit 43, which has been enabled to respond to the "lock" pulse by the"1" level of the "step freeze" signal, causes the AFT signal to beapplied to LPF 35. In response to the AFT signal, the LO frequency iscontinuously changed to reduce any frequency deviation of a picturecarrier from its nominal value, e.g., in the United States 45.75 MHz.

If there is no picture carrier located for the nominal LO frequencystep, the AFT signal will cause the LO frequency to drift beyond thepredetermined offset, e.g., 1.25 MHz, detected by offset detector 47 andan "offset" signal will be generated. The "offset" signal is coupled tomode control unit 43 and, as a result, reestablishes PLL operation. Inaddition, an "and" gate 91 enabled by the "1" level of the "step freeze"signal couples the "offset" signal to the input to another "and" gate93. The purpose of "and" gate 93 will be described below. For now, it isonly necessary to understand that "and" gate 93 is enabled at this pointand thereby couples the "offset" signal to the clock (C) input of 1 MHzstep control counter 57. This causes counter 57 to change from the statecorresponding to a count of 6 to a state corresponding to a count of 8and the LO frequency is changed to a step +1.0 MHz from its nominalvalue. After the "lock" pulse is generated, the AFT signal is againcoupled to LPF 35. If the "offset" signal is again generated, PLLoperation is once again initiated and the state of 1 MHz step controlcounter 57 is changed to correspond to a count of 4. This causes the LOfrequency to set to the -1.0 MHz step.

Thereafter, the tuning system switches between AFT and PLL controloperations in the above-described manner for successive frequency stepsof +2 MHz, -2 MHz and -3 MHz until a picture carrier is tuned during anAFT control operation as manifested by the absence of an "offset"signal. If no picture carrier for the selected channel is present, an"offset" signal will be generated during the -3 MHz step. In response, 1MHz step control counter 57 is once again set to a state correspondingto a count of 6, i.e., the nominal LO frequency value and the AFTcontrol operation is cyclically initiated when the "lock" pulse isgenerated. This continues until a picture carrier is provided for theselected channel or a new channel is selected. The nominal LO frequencystep is selected while waiting for a picture carrier under theassumption that many, if not most, RF carriers are near their standardfrequencies.

After the nominal LO frequency step is established, 1 MHz step controlcounter 57 is inhibited from being changed further. Specifically, whencounter 57 completes one complete counting cycle, a "1 cycle" signalhaving a "1" level is generated. In response, an inverter 95 generates a"0" which disables "and" gate 93 and thereby prevents the "offset"signal from being coupled to the clock (C) input of counter 57.

Counter 57 may comprise a three stage Johnson counter which produces 6states and a decoder for converting the binary output signals of eachstage produced at each state into a respective count-representativedigital word according to the following logic truth table.

    ______________________________________                                        JOHNSON COUNTER          LO FREQUENCY                                         STATE           COUNT    STEP                                                 ______________________________________                                        111             6         0                                                   011             8        +1                                                   001             4        -1                                                   000             10       +2                                                   100             2        -2                                                   110             0        -3                                                   ______________________________________                                    

In this case, to detect the completion of one cycle an "and" gate may beused to detect the 111 state and to set a first flip flop in response tothe first generation of that state and a second flip flop in response tothe second generation of that state.

If a picture carrier has been located and tuned in either the synctransition search of AFT search operations, after a predetermined time,e.g., 3 seconds, the step at which the tuning occurred is maintainedeven if the "offset" signal is thereafter generated because the RFsignal is temporarily lost. To that end, a counter 97, receiving clockpulses from fixed divider (÷R) 31 at its clock (C) input and reset inresponse to the "change" pulse coupled to its reset (R) input, generatesa positive-going pulse three seconds after a new channel is selected. Aninverter 99 inverts the positive-going pulse signal to produce anegative-going pulse three seconds after a new channel is selected. Thatnegative-going pulse is coupled to "and" gate 93 to disable it andthereby prevent "offset" signals from changing the contents of 1 MHzstep counter 57. Prior to the pulse generated three seconds after a newchannel is selected, the output signal is a "1" thereby enabling "and"gate 93 to respond to the "offset" signal. Accordingly, if a picturecarrier has been tuned for less than three seconds but is lost, thecontents of 1 MHz step counter 57 are changed in response to the"offset" signal as described above.

In the same vein, the negative-going pulse generated by inverter 99three seconds after a new channel has been selected disables an "and"gate 101 from coupling the "offset" signal to the reset (R) input ofsync presence transition detector 63. Prior to that time, "and" gate 101is enabled to couple the "offset" signal to the R input of detector 63.In response, the "step freeze" is generated. This allows stepping tocontinue from the value previously set during the sync presencetransition search mode as previously described.

It is noted that a step size of 1 MHz rather than 0.5 MHz is used duringthe AFT search mode since the continuous control provided by the AFTsignal makes up for the difference in step size resolution.

The logic arrangement for pulse swallower 41 shown in FIG. 4 includes D(data) type flip-flops 401 and 403 arranged so that only one outputpulse of the output signal of fixed divider (÷K) 25 is swallowed inresponse to the falling edge each BRM output pulse as indicated in thetiming diagram shown in FIG. 4a.

The logic for sync presence detector 63 is shown in FIGS. 5a and 5b andits timing diagram is shown in FIG. 5c.

In FIG. 5c, if the composite synchronization (sync) signal generated bysync processing unit 11 is assumed to have nominal NTSC parameters, thesynchronization pulses have a width of 4 microseconds and a period of63.5 microseconds. Sync presence detector 63 examines the pulse width,period and noise content of the composite sync signal to determine if itis correct.

To that end, the composite signal, e.g., having excursions between +5volts and ground potential, is coupled to the non-inverting (+) inputsof threshold comparators 53 and 55. A relatively high reference voltage,e.g., +4 volts, just below the maximum pulse amplitude, is applied tothe inverting (-) input of comparator 53. A relatively low referencevoltage, just above the minimum pulse amplitude, e.g., +1 volt, isapplied to the inverting (-) input of comparator 55. Absent noisetransients in the composite sync signal, each of the output signals ofcomparators 53 and 55 will include a number of pulses equal to thenumber of pulses in the composite synchronization signal. However, ifundesired negative-going noise transients are present, these will resultin extra pulses in the output signal of comparator 53. Similarly, ifpositive-going noise transients are present, there will be extra pulsesin the output signal of comparator 55.

The output signal of comparator 53, hereinafter called SYNC-H, iscoupled through an inverter 57 to the reset (R) input of a binary ripplecounter 59 comprising, e.g., a CD4024 integrated circuit (IC) availablefrom RCA Corporation, Somerville, N.J. A reference signal with a 4microsecond period is applied to the clock (C) input of binary counter59. The Q1 and Q2 output signals are applied to a "and" gate 511. Inessence, this allows the number of 4 microsecond clock pulses to beroughly measured during the SYNC-H pulse width. (It is noted that sincethe 4 microsecond clock pulses are not synchronized with the SYNC-Hpulses, this width measurement is only an approximate one. However, ithas been found to be effective to detect most, if not all, widtherrors.) If the pulses have a pulse width less than 8 microseconds,since the clock signal is only 4 microseconds long, the pulse width istaken as being correct and a count of 3 will not be reached.Accordingly, the output of "and" gate 511 will be at a low logic level("0"). However, if the SYNC-H pulses have a width greater than 8microseconds, a count of 3 will be reached and the output of "and" gate511 will be a high logic level ("1").

The SYNC-H signal is also applied to a period counter 513a.Specifically, this SYNC-H signal is applied to the clock (C) input of abinary ripple counter 515, which may also comprise a CD4024 IC, alongwith a reference signal R2 as a reset signal to the reset (R) input. Ifthe frequency of the SYNC-H signal is excessive, e.g., due to thepresence of noise transients, counter 515 will reach a count of 3 and"and" gate 517 will produce a "1". A set-reset flip-flop 519 is set inresponse to R2. If the frequency of the SYNC-H signal is low, counter515 will not reach a count of "1" and SR FF 519 will remain a "1". Theoutput signal of flip-flop 519 is gated at the rate of a referencesampling signal R1 through a gating circuit 521 to a set (S) input of SRFF 523. The output of "and" gate 517 is also applied to an S input of SRFF 523. The output signal of "and" gate 511 is also applied to an Sinput of SR FF 523. Another frequency counter 513b, similar to 513a andhaving its output also coupled to an S input of SR FF 523, is responsiveto the SYNC-L signal produced at the output of comparator 55 andtherefore determines when its frequency is incorrect.

If any S inputs of SR FF 523 is a "1" before a reference signal R2,which is applied to the reset (R) inputs of SR FFs 519 and 523, occurs,a clock pulse is applied to a clock (C) input of an error counter,comprising a binary ripple counter 525, such as a CD4040 IC, when R2 isa "1".

The number accumulated by error counter 525 during a 16,384 microsecondmeasurement interval established by a timing circuit 527 determineswhether the composite synchronization signal is correct or not. Inresponse to the Q4 and Q5 output signals of binary counter 525, a first"and" gate 529 sets a first SR FF 531 when the number of errors is 24 ormore. In response to the Q6 and Q7 output signals, a second "and" gate533 sets a second SR FF 535 when the number of errors is 96 or more. Theoutput signals of FFs 531 and 535 are applied through a "nor" gate 537to the D input of a D FF 539. When a positive edge of the output signalof timing circuit 527 is generated, if either one of the input signalsof "nor" gate 537 is a "1", the Q output signal of FF 539, i.e., the"sync presence" signal, is set to a "0", indicating an incorrect syncsignal. Conversely, if neither one of the input signals of "nor" gate537 is a "1", the Q output signal of FF 539 is set to a "1", indicatinga correct composite synchronization signal.

The Q and Q output signals of FF 539 are fed back to the reset inputs ofFFs 531 and 535 to prevent the output signals of FF 539 from changingstate from one timing interval to the next when the error count isbetween 24 and 96. This provides hysteresis to prevent sporadictransitions in the output signals of FF 539.

FIG. 5b shows a logic arrangement for generating the various timingsignals referred to above. It includes two cascade binary ripplecounters 541 and 543 comprising, e.g., CD 4040 and CD4024 ICs,respectively, and desirably located in fixed divider (÷R) 31. Referenceoscillator is selected to provide a 4 MHz clock (C) signal for counter541. The remaining portion of the timing circuit includes an inverterand two "and" gates combining the output signals of counter 541 togenerate timing signals R1 and R2. The timing diagrams of FIG. 5b alsoshow the relationship between the composite sync pulses of variousfrequencies and the measurement intervals between R2 pulses and betweenR2 and R1.

The logic arrangement for sync presence transition detector 63 shown inFIG. 6 includes three D FFs 601, 603 and 605. The "lock" pulse isapplied to the clock (C) input of these FFs. The "sync presence" signalis applied to the data (D) input of D FF 601 and Q output of D FF 601 iscoupled to the D input of D FF 603. Accordingly, the sync presence levelexisting when a first "lock" pulse is generated is entered into D FF 601and entered into D FF 603 in response to a second "lock" pulsegenerated. The sync presence level existing when the second "lock" pulseis generated is entered into D FF 601. If, at the time a third "lock"pulse is generated, the Q output of D FF is a "1" and the Q output is a"1" (which corresponds to a "01X" condition), an "and" gate 607 couplesa "1" to the D input of D FF 605 and its Q output is set to a "1" as the"step freeze" signal. Because three "lock" pulses are needed before the"step freeze" signal is generated, the LO frequency is decreased by anadditional 0.5 MHz step as described above. Sync presence transitiondetector 63 is set and reset in response to the operation of "or" gate67 and "and" gate 101 as described above.

While the present tuning system has been described as being useful toaccurately locate and tune those channels having modulated picturecarriers as well as locating and tuning those, such as music channels,with unmodulated carriers, it is also useful when video accessories,such as camera, cassette and disc players are the RF source. This is sobecause when such accessories are in a standby mode, the picture carrieris unmodulated.

While the present invention has been described with reference to apreferred embodiment, modifications are intended to be within the scopeof the invention defined by the following claims.

What is claimed is:
 1. In a television receiver including RF means forselecting an RF signal from among a plurality of RF signals associatedwith respective channels which may be selected, local oscillator meansfor generating a local oscillator signal, mixer means for generating anIF signal by heterodyning said selected RF signal and said localoscillator signal, automatic fine tuning (AFT) means responsive to saidIF signal for generating an AFT signal representing the deviationbetween the frequency of the picture carrier of said IF signal and itsnominal value, and synchronization processing means for generating apicture synchronization signal in response to said IF signal, apparatuscomprising:tuning control means for controlling the frequency of saidlocal oscillator signal including step means for changing the frequencyof said local oscillator signal in steps; first search means coupled tosaid step means and responsive to said synchronization signal forcausing said step means to change the frequency of said local oscillatorsignal in steps from a first value toward a second value in a firstfrequency range until said synchronization signal has a predeterminedcondition or the frequency of said local oscillator signal has saidsecond value; second search means coupled to said step means andresponsive to said AFT signal for causing said step means to change thefrequency of said local oscillator signal in steps from a third valuetoward a fourth value in a second frequency range substantially the sameas said first range until said AFT signal has a predetermined conditionor the frequency of said local oscillator signal has said fourth value;and search mode control means for first enabling the operation of saidfirst search means and thereafter enabling the operation of said secondsearch means if the frequency of said local oscillator signal reachessaid second value under the control of said first search means.
 2. Theapparatus recited in claim 1 wherein:said step means changes thefrequency of said local oscillator signal in steps smaller than thefrequency separation between adjacent channels.
 3. The apparatus recitedin claim 2 wherein:said first and second ranges are smaller than thefrequency separation between adjacent channels.
 4. The apparatus recitedin claim 2 wherein:said tuning control means includes switch means forselectively coupling said AFT signal to said local oscillator means;said second search means is coupled to said switch means for causingsaid switch means to couple said AFT signal to said local oscillatormeans after each step when said second search means is enabled tooperate; and said second search means includes offset detector meansresponsive to said local oscillator signal for generating, after eachstep, an offset signal when said second search means is enabled tooperate if the frequency of said local oscillator signal is caused to beoffset from the value at the last step by a predetermined amount, saidsecond search means causing said step means to change the frequency ofsaid local oscillator signal to the next step if said offset signal isgenerated.
 5. In a television receiver including RF means for selectingan RF signal from among a plurality of RF signals associated withrespective channels which may be selected, local oscillator means forgenerating a local oscillator signal, mixer means for generating an IFsignal by heterodyning said selected RF signal and said local oscillatorsignal, automatic fine tuning (AFT) means responsive to said IF signalfor generating an AFT signal representing the deviation between thefrequency of the picture carrier of said IF signal and its nominalvalue, and synchronization processing means for generating a picturesynchronization signal in response to said IF signal, apparatuscomprising:tuner control means for controlling the frequency of saidlocal oscillator signal in response to the selection of one of saidchannels including step means for changing the frequency of said localoscillator signal in steps; first search means coupled to said stepmeans for causing said step means to change the frequency of said localoscillator signal in steps from a first value toward a second value in afirst frequency range less than the frequency spacing between channels;said first search means including synchronization signal detector meansresponsive to said synchronization signal for generating, after eachstep, a stop signal if said synchronization signal exhibits apredetermined condition, said first search means causing said step meansto change the frequency of said local oscillator to the next step ifsaid stop signal is not generated until the frequency of said localoscillator signal reaches said second value; second search means coupledto said step means for causing said step means to change the frequencyof said local oscillator signal in steps from a third value toward afourth value in a second frequency range also less than said frequencyspacing but overlapping a substantial portion of said first frequencyrange; said second search means including switch means for selectivelyapplying said AFT signal to said local oscillator after each step; saidsecond search means also including offset detector means responsive tosaid local oscillator signal for generating, after each step, an offsetsignal if the frequency of said local oscillator signal is caused to beoffset from the value at the last step by a predetermined amount, saidsecond search means causing said step means to change the frequency ofsaid local oscillator signal to the next step if said offset signal isgenerated until the frequency of said local oscillator signal reachessaid fourth value; and search mode control means for first enabling theoperation of said first search means when a channel is selected andthereafter enabling the operation of said second search means if thefrequency of said local oscillator signal reaches said second valueunder the control of said first search means.
 6. The apparatus recitedin claim 5 wherein:said first search means is coupled to said switchmeans for causing said switch means to couple said AFT signal to saidlocal oscillator means if said synchronization signal has saidpredetermined condition.
 7. The apparatus recited in claim 6 wherein:foreach of said channels, the frequency of said local oscillator signal hasa nominal value; and said first value is a positive value with respectto the nominal value for the selected channel and the second value is anegative value with respect to the nominal value for the selectedchannel.
 8. The apparatus recited in claim 7 wherein:said second searchmeans causes said step means to change the frequency of said localoscillator signal to the nominal value for the selected channel, thenominal value being said third value, when first enabled to operate andagain if said fourth value is reached.
 9. The apparatus recited in claim8 wherein:said second search means causes said step means to change thefrequency of said local oscillator signal by alternate positive andnegative steps with respect to the nominal value for the selectedchannel, in the order named, in response said offset signal.
 10. Theapparatus recited in claim 9 wherein:said second search means causessaid step means to change the frequency of said local oscillator signalby larger steps than does said first search means.
 11. The apparatusrecited in claims 4 or 5 wherein:said local oscillator means isresponsive to a tuning signal for controlling the frequency of saidlocal oscillator signal; said tuning control means includes locked loopmeans responsive to said local oscillator signal for generating saidtuning signal to establish a proportional relationship between thefrequency of said local oscillator signal and a reference frequency; andsaid step means selectively changes the proportional relationshipbetween the frequency of said local oscillator signal and said referencefrequency under the control of said first and second search means. 12.The apparatus recited in claim 11 wherein:said locked loop meansincludes a source of a reference frequency signal the frequency of whichis equal to said reference frequency; programmable divider means fordividing the frequency of said local oscillator signal by a controllablefactor to produce a frequency divided version of said local oscillatorsignal; and comparison means responsive to said reference frequencysignal and said frequency divided version of said local oscillatorsignal for generating said tuning control signal to cause the frequencydivided version of said local oscillator signal to substantially equalsaid reference frequency; and said step means selectively changes thecontrollable factor under the control of said first and second searchmeans.